This invention relates to methods and devices for illuminating a surface and obtaining an image of the surface, and particularly to hand-held illumination and imaging devices for the purpose of reading labels having information in a code, such as a bar code, or an alphanumeric format.
Devices for imaging of surfaces provide an electronic representation of an image of the surface of an object. The representation in electronic form is then stored or transmitted to an appropriate data processing apparatus. If information is stored in the label in alphanumeric format, the image would be transmitted to a data processing apparatus having character recognition capacity. Typically, however, routine information is printed in a code format. A common example is a bar code label affixed to the surface of a package. The bar-code label contains information about the package's origin, destination, order number, and the like. To retrieve the information from the bar code label, an appropriate imaging device electronically creates a digitized image of the bar code label. That image is then transferred to a processing computer. The processing computer deciphers the desired information from the digitized image.
An imaging device can be either machine-mounted or hand-held. Portable hand-held devices are especially useful when processing is required at remote locations. Such devices are also useful when the objects to which labels have been applied may be in a variety of sizes or positions.
The ability to retrieve data from the surface of an object depends largely on the quality of the digital image obtained by the imaging device. Glares, shadows and non-uniformity in illumination on the surface during the imaging process result in degradation of the digital image. More specifically, image processing software may be unable to reliably identify the optical characteristics (such as color) of every portion of the label. As a result, data will be lost. Uniform illumination over a surface is especially important when reading information on an alphanumeric label or in a two-dimensional encoded format where there is a minimum of redundancy.
Non-uniform ambient light is one source of non-uniformity in illumination. A second problem, which is particularly common in shipping of packages and letters, results from the practice of placing a shiny protective layer, such as transparent tape, over the label. Such tape will cause specular reflection from the surface, which will obscure the image of the underlying label.
A desirable goal in obtaining images of labels is to obtain the image as quickly as possible. This goal makes it desirable to be able to capture the entire image of a two-dimensional label substantially instantaneously.
The use of charge-coupled device based cameras has been proposed for obtaining images of labels. However, CCD-based cameras are subject to the drawback that the photo-charges in the potential wells in the CCD array can exceed the capacity of the potential wells in the presence of intense illumination. Charges spill over into adjacent potential wells, which phenomenon is known as blooming. Blooming, of course, results in loss of information.
Furthermore, relative motion of the camera and the substrate can cause blurring of the obtained image. For example, a normal video exposure time is 0.033 seconds. In that time, an object moving at 5 inches per second moves 0.165 inches, which, in 100 dpi image resolution, is 16.5 pixels. Such a pixel shift will provide a very blurred image, with resulting loss of data.
It is an object of this invention to provide a method and apparatus for illumination and imaging of a surface which obtains accurate images in a short amount of time.
It is a particular object of this invention to provide a method and apparatus for illumination and imaging of a surface which employs a CCD-based camera which is not subject to blooming or to blurring.
It is a further particular object of this invention to provide an apparatus and method for illumination and imaging of a surface which is not subject to specular reflection from the surface.
Additional objects and advantages of the invention will become apparent from the detailed description of a preferred embodiment which follows.